gait classification

There is a sector of our community that sees classification as the holy grail of gait analysis. If only we could divide our patients into neat little categories then our problems will be solved. But why do we want to classify patients?

The most obvious reason for categorising patients into groups would be if genuinely different groups existed. But do they? In an earlier blog I’ve written about the GMFCS and pointed out that there is absolutely no reason to believe that children with cerebral palsy fall into distinct categories of severity – they almost certainly lie on a continuous spectrum. I suspect that the same is true of gait patterns. Indeed in work that Fiona Dobson completed for her PhD thesis but never published we used a technique called multi-dimensional scaling to look for evidence of clustering of gait patterns in children with hemiplegic cerebral palsy. We couldn’t find any. Whichever way we looked at the data it looked like a random scatterplot.

So if there is so little evidence that gait patterns fall into distinct categories why are we so obsessed by it?

I suspect that classification systems are assumed to help clinicians – particularly less specialist ones than are employed in our gait analysis services. If we can simplify understanding of gait to a small number of discrete categories then we’ll be able to teach people to recognise these. If we can describe management plans for the different categories then maybe we can even teach people to treat the patients.

There are problems though. The first one is that the classification systems don’t seem to work – clinicians can’t agree on what category a gait pattern should be placed in. Fiona Dobson, in one of the few studies to assess reliability of a classification in a centre outside that in which it was developed (Dobson et al,2007), looked at the agreement of 16 clinicians in rating gait patterns of 34 children with hemiplegic cerebral palsy . On the basis of video data, only one pattern was put in the same category by all clinicians and in over a third there was less than 50% agreement. An important feature of this study was that the 16 clinicians were all specialists in gait analysis of children with cerebral palsy. What would the results have been like if the non-specialist clinicians, who presumably have most to gain from classification systems, had been recruited to the study? Results were a little better when gait analysis data was used as a basis for classification, but then the clinicians for whom classification is likely to be most useful don’t generally have access to gait analysis.

Another issue is that gait patterns are almost certainly a combination of characteristics of the child and of how they have been managed previously. The Winters, Gage and Hicks (1987) classification of hemiplegia intentionally excluded children who had had previous surgery but the classification of diplegia by Rodda et al. (2004) included children with prior calf surgery (because there were so many of them). We never quantified it but it appeared to us that fewer and fewer children in Melbourne fell into the Rodda “crouch” category over time which we attributed to more conservative early surgical management and the wider use of Botulinum toxin. If this is the case then classifications schemes will have to be sufficiently generalizable to cover the effects of different management practices or maybe we need a number of different classification schemes depending on different management practices in different parts of the world.

Developing and validating classification schemes is a considerable undertaking. Earlier schemes were generally based on the enlightened but essentially subjective opinions of leading clinicians. In an era of evidence-based practice this is no longer satisfactory and widespread consultative processes (such as a Delphi process) would now be needed to convince the clinical community of any new scheme. Similarly were existing schemes have been validated this has tended to be on the basis of repeatability studies based within the teams who developed the scales and typically using gait analysis techniques that may not be available to those who are most likely to benefit from the classification. Convincing validation of new schemes will need to be more comprehensive. Such an investment of time and effort would be perfectly justified if we are certain that the schemes will be useful – but we do need to be certain.

Validation is made considerably more complex by the fact that the classification systems do not fit the underlying reality. How can you interpret reliability studies designed to test the competence of clinicians to ascribe gait patterns to categories when there is genuine ambiguity in whether any individual child actually fits a particular category? A similar issue arises with the usefulness of the scheme for less experienced clinicians. Many children will either be on the borderline of categories or may not fit cleanly into categories at all. Explaining how to deal with such exceptions to generalist staff may lead to more confusion not less.

So is there an alternative? Well I’d like to float the potential of what I call profiling. Rather than select a number of discrete groups into which gait patterns can be assigned we define aspects of the gait pattern that vary across the patient population. We can then score or grade the different aspects separately. This allows us to accept that there is a continuous distribution of gait patterns and that also that gait might vary independently across the different aspects rather than suggesting that there is necessarily grouping of the aspects as is required by classification schemes.

We presented a paper proposing this for the gait of children with hemiplegic cerebral palsy at the Sydney CP meeting in 2008 (Tirosh, Dobson et al.) but never took the ideas any further. A factor analysis based on movement analysis profile scores suggested that 85% of the variability could be explained by 5 factors which reflected:

These factors all make clinical sense as does the idea that hemiplegic gait should be categorised by assessing how far each of these different aspects contributes to the gait pattern of an individual child rather than trying to assign the gait pattern to one of a small number of categories. Perhaps more importantly it appears to me (and I admit this is a subjective opinion) that the underlying model of gait patterns varying along a continuum in a number of different dimensions matches reality more closer than a model assuming that gait patterns fall into distinct and recognisable categories.